Description

Prior to this patch, Overlay appends "overlay" to all names in the Font, XObject and ExGState resource dictionaries, then rewrites content stream(s) in the overlay PDF to reference those new names using a simple hand-rolled content-stream find-and-replace process. It doesn't check for over-length names, and it doesn't check to make sure that the newly generated name(s) don't clash. Because PDFs often use the same names for objects, this quickly becomes a problem when you're doing multiple overlays - something that becomes more likely with https://issues.apache.org/jira/browse/PDFBOX-1255 but is already useful to do with stock PDFBox.

This patch alters Overlay so that it only renames objects from the overlay PDF when there is a conflict with the PDF being overlaid upon. It also uses a name generation strategy that checks for conflicts and for over-length names, so multiple overlays will work much better. The patch uses the PDFStreamProcessor (a simplified base extracted from PDFStreamEngine by https://issues.apache.org/jira/browse/PDFBOX-1256) to copy each stream from the PDF to overlay to a ContentStreamWriter. It checks for names that reference renamed resources and substitutes the new name before writing each operator and its arguments to the output stream.

The main benefit of this patch is that it enables multiple overlays without name clashes.

A secondary benefit of this patch is that it eliminates Overlay.java -specific code in favour of using facilities provided by the rest of PDFBox. That makes Overlay a better example, helps it test the rest of PDFBox better, and makes it benefit from improvements in PDFBox's stream processor and writer.